Method of casting metals



United States Patent 3,153,824 METHQD 0F CASTlNG Tunis L. Holmes, Deer-field, ill, assignor to Metals Corporation, a corporation of Delawme N0 Drawing. Filed Dec. 29, 1961, Ser. No. 363,927 16 Claims. (Cl. 22l92 This invention relates to the casting of metals and alloys. More particularly, it relates to a method of melting metals and casting the molten metals in refractory molds under the influence of vacuum.

Briefly, the invention comprises a method of casting metals wherein the metallic element or elements are melted under relatively high vacuum and cast in refractory molds subjected to a pretreatment at a temperature in excess of the approximate melting temperature of the metal composition being cast at a relatively high vacuum for a time sumcient to expel substantially all of the volatiles which would be liberated by contact of molten metal with an untreated casting mold or a mold heated to conventional preheat temperatures.

Vacuum casting, as presently used for the melting and pouring of metal, either a single metallic element or combinations of elements (alloys), is carried out in two general types of apparatus. In both types, the apparatus comprises a sealed melting or furnace chamber containing a crucible equipped with suitable heating means. The melting chamber is connected to a vacuum pump and is provided with suitable means for manipulation of the crucible so that the crucible may be tilted or otherwise positioned to pour the molten metal. In one type of apparatus, a mold adapted to receive the molten metal is mounted within the furnace chamber.

It is the common practice in the use of such apparatus for vacuum casting of relatively high melting point metals or alloys to preheat the mold to a temperature between 1000 F. and 24-00" F. and after pouring molten metal into the mold heated to the preheat range, to rapidly cool the mold to solidify the casting.

Because of the need for reliability, application designers, in general, work to the low limit of the scatter band of properties of a casting.

The difierence between the high limit and the low limit of a property of a casting is often appreciable. For example, the difference between the temperature at which an alloy is known to be capable of operating and the temperatures specified by application designers as a safe operating temperature for castings of that alloy may vary significantly. If the diiference between the limits can be reduced by elimination of conditions conducive to the production of castings which have properties at or approaching the low limit, cast components can be designed to approach more closely to the maximum of properties which an alloy is inherently capable of exhibiting. Cast ings prepared by the methods currently in use have been only partially successful in eliminating the defect conditions of porosity and casting shrinkage which heretofore have resulted in broad scatter bands of properties and required the downgrading of alloys in the interest of reliability.

Porosity, either micro or macro, is a condition which can exist at a casting surface or can extend into the interior of the casting. One form of common defect is surface roughness or craters. Another form may be "ice present as internal voids which will serve as stress points for initiation of cracks.

The shrinkage problem also has its external and internal manifestations. Such manifestations in the turbine blade and vane field, when internal, are commonly referred to as root shrinkage and centerline shrinkage.

Castings prepared by the process hereinafter described in detail, are characterized by a marked reduction in the above mentioned type of defects and significantly increased reliability.

Now i have discovered that when metals are melted under conditions of relatively low subatmospheric pressure (high vacuum) and a refractory mold is heated, under the vacuum conditions maintained in the metal melting chamber, to a temperature in excess of the melting temperature or" the metal composition and preferably to the pouring temperature of the metal composition for a period of time necessary to evacuate substantially all of the volatiles from the mold, usually 5 to minutes, and then the heated refractory mold is cooled to a temperature below the temperature of the molten metal, conditions are created whereby molten metal being poured into the preheated mold will produce a casting which exhibits proper fillout, feeding from the sprues, etc., and is substantially free of porosity and shrinkage defects. In addition, the mechanical and structural properties are substantially improved.

In accordance with my invention, a suitable mold capable of withstanding relatively high temperatures without distortion or slumping is pretreated under conditions of temperature and vacuum, for example, approximately equal to those to which the molten metal is subjected at the time of pouring, so that the mold is rendered a substantially inert body at the temperature and vacuum conditions which will afiect it when the molten metal is poured into the mold cavity. Prior to the pouring, however, the preconditioned refractory mold is cooled to a temperature of 200 F. to 1000 F. below the pouring temperature of the molten metal to establish an appropriate temperature differential so that, during casting, a satisfactory solidification gradient can be developed. Molten metal is poured into the cooled pretreated mold being maintained under the influence of the high vacuum and the casting is held under vacuum until solidification. After solidification, cooling of the casting and mold may be completed under atmospheric conditions, following which the casting is processed in the usual manner of knockout, cut off, and finishing grinding.

in a preferred embodiment of the invention, a mold suitable for use in the casting process of this invention is made from a material which is substantially nonreactive at temperatures up to about 600 F. above the melting point of the metal to be cast. Highly refractory, relatively inert materials such as quartz, zirconia, and the like, have been found to meet these requirements.

A mold formed from suitable non-reactive materials and produced with suitable sprues, gates, and the like, is subjected to treatment in a chamber maintained at a vacuum of less than 20 microns and preferably a chamber maintained at a vacuum of in the range between 0.01 and 5 microns.

The nature of the mold, the nature of the associated mold support, if any, and the temperature to which the mold is heated, all have a definite influence upon the 'form size.

length of time required for the vacuum treatment. If the mold has been prepared by the multiple deposition method with, for example, zirconia particles of progres sively greater size as the thickness of the mold is increased, the mold will have porosity characteristics lending itself to a short time vacuum treatment.

Preconditioning of a mold to the point where volatiles will not be expelled from the mold into the metal being poured into the cavity thereof, requires that the refractory mold be heated to a temperature above the melting point of the metal to be poured, and also that the entire mold be brought to the desired temperature. Naturally, the time of heating varies with the thickness of the mold.

The temperature to which the refractory mold should be raised in the preconditioning operation will vary with the melting point and more generally the pouring temperature of the molten metal, which pouring temperature is usually between 100 F. and 600 F. above the melting point temperature for the material being rendered molten. Generally, in the pouring of metal castings designed to operate at high temperatures, for example, blades or vanes of gas turbine engines, the alloys melt at a temperature in the range between about 2400 F. and 2700 F. and are poured at temperatures in the range between about 2600 F. and 3000 F.

Refractory shells or molds, when at the above described melting or pouring temperatures of the metal to be cast, are not in a proper condition for reception of molten metal. A preheated mold, in addition to being held under vacuum should be cooled to a temperature in the range between about 1000 F. and 2400 F. and preferably to a temperature in the range between about 1800 F. and 2300 R, if the pouring temperature of the metal a is between about 2400" F. and 2800 F. Castings poured when the mold is at an appropriate temperature, exhibit a proper fillout and adequate feeding so that the defects known as porosity shrinkage, etc., will be minimized or eliminated.

Castings prepared in accordance with the method of this invention, have a grain structure, as determined, for example, by macroetching, of a large and relatively uni- T he strength of these castings may be partly attributable to the presence of this grain structure for it is recognized by those skilled in metallurgy that large grain size favors development of high temperature strength. The method of treating the mold and casting the alloys described herein apparently minimizes the number of particles capable of acting as nucleating agents from which grains begin their growth, thus facilitating "the development of large grain size. a

One form of apparatus for carrying out the casting of alloys in accordance with this invention, consists of a vacuum chamber having a melting furnace, a mold heater and a pumping system communicating with the interior of the chamber adapted to evacuate gases from the chamber. Associated with the vacuum chamber is a mold charging lock provided with a lift for elevating the ceramic mold into the pouring position within the mold heater.

In order to more fully illustrate the invention, the following examples are included. These examples are intended to be illustrative only and are not to be construed as limitations on the scope of the invention.

Example I zirconium, 0.85% carbon, 0.005% boron, 1% iron, and

the balance cobalt.

The vacuum chamber is closed and the chamber evacuated using a vacuum pump capable of maintaining the pressure within .the chamber at less than 2 micron. The

metallic elements are reduced to molten form by heating to approximately 2550 F.

A zircon mold formed by the deposition of a multipl city of layers of zircon is moved by means of an elevator through the mold lock into a position within the mold heater consisting of a graphite induction susceptor and a surrounding induction coil. The mold is heated to approximately 2800 F. and held at approximately that temperature for 30 minutes while a vacuum of less than 2 micron, as measured by a Philips gauge, is maintained. After holding the mold for the above-stated period of time at temperature, the mold is allowed to cool to approximately 1900 F.

After the mold is allowed to cool to about 1900 F., the molten alloy is poured into the mold. When the pouring operation is complete, the casting is held in the vacuum chamber for about 5 minutes, following which the mold is removed from the chamber through the mold lock and cooled to room temperature in an air atmosphere.

The cast test bars of cobalt-base alloy have a smooth surface substantially free of pits, craters and porosity. A study of the internal structure of the cast bar shows freedom from internal defects. Stress rupture tests on bars prepared in Example I exhibit a rupture life of 40 to 50 hours at test conditions of 2000 F. and 8000 psi. The same metal cast under standard vacuum procedures heretofore in use and tested under these same conditions shows an average rupture life in the range between 20 and 30 hours.

Cast turbine vanes of the cobalt base alloy described in Example I show, in addition, other improved characteristics which differentiate them from vanes cast by the common vacuum casting procedures. The vanes, for example, when subjected to thermal shock testing conditions of alternately heating to high temperature for two minutes followed by abrupt cooling for one minute, withstood 600 cycles at 2000 F. plus an average of 900 cycles at 2100 F. before cracking. This superior thermal shock characteristic of alloys cast in accordance with applicants invention is to be compared with an average of approximately 500 cycles at 2000 F. to cause cracking in alloys cast in accordance with common vacuum casting procedure.

Example 11 Metallic elements are introduced into an induction melting furnace within an evacuable chamber in quantities to form an alloy having the following composition:

14% chromium, 4.5% molybdenum, 2% columbium, 6.0% aluminum, 0.01% boron, 0.15% carbon, 1% titanium, 0.8% zirconium, and the balance nickel.

The vacuum chamber is closed and the chamber evacuated using a vacuum pump capable of maintaining the pressure within the chamber at less than 1 micron. The metallic elements are reduced to molten form by heating to approximately 2520 F.

A zircon mold formed by the deposition of a multiplicity of layers of zircon is moved by means of an elevator through the mold lock into a position within the mold heater consisting of a graphite induction susceptor and a surrounding induction coil. The mold is heated to approximately 2600 F. and held at approximately that temperature for 20 minutes while a vacuum of less than 2 micron, as measured by a Philips gauge, is maintained. After holding the mold for the above-stated time at temperature, the mold is cooled to approximately 1900 F.

After the mold has been allowed to cool to about 1900 F., the molten alloy is poured into the mold. When the pouring operation is complete, the casting is held in the vacuum chamber for about 30 minutes, following which the mold is removed from the chamber through the mold lock and cooled to room temperature in an air atmosphere.

The cast test bars of nickel-base alloy have a smooth surface substantially free of pits, craters and porosity.

A study of the internal structure of the cast bar showed freedom from internal defects. Stress rupture tests on bars prepared in accordance with Example 11 exhibit a rupture life at 1800 F. and 22,000 p.s.i. test conditions of 40 to 50 hours. The same alloy cast under standard vacuum procedures heretofore in use and tested under these same conditions of 1800 F. and 22,000 p.s.i. shows an average rupture life in the range between 25 and 30 hours.

Example 111 Metallic elements are introduced into a magnesium oxide crucible within an evacuable chamber in quantities to form an alloy having the following composition:

0.08% carbon, 14.7% chromium, 26.3% nickel, 1.1% manganese, 0.44% silicon, 1.2% molybdenum, 2.05% titanium, 0.15% vanadium, 0.22% aluminum, and the balance iron.

The vacuum chamber is closed and the chamber evacuated using a vacuum pump capable of maintaining the pressure within the chamber at less than 1 micron. The metallic elements are reduced to molten form by heating to about 2640 F. by means of an induction coil surrounding the magnesium oxide crucible.

A zircon mold formed by the deposition of a multiplicity of four layers of zircon is moved by means of an elevator through the mold lock into a position within the mold heater consisting of a graphite induction susceptor and a surrounding induction coil. The mold is heated to approximately 2850 F. and held at approximately that temperature for 30 minutes while a vacuum of approximately 1 micron, as measured by a Philips gauge, is maintained. After holding the mold for the above-stated time at temperature, the heat was reduced so that the mold cooled to approximately 1900 F.

After the mold has been allowed to cool to about 1900 F., the molten alloy is poured into the mold. When the pouring operation is complete, the casting is held in the vacuum chamber for about 5 minutes, following which the mold is removed from the chamber through the mold lock and elevator and cooled to room temperature in an air atmosphere.

The test bars of iron-base alloy cast have a smooth surface substantially free of pits, craters and porosity. A study of the internal structure of the cast bar showed substantial freedom from internal defects. Stress rupture life tests on bars prepared in accordance with Example 111 exhibit a rupture life at 1200 F. and 65,000 p.s.i. test conditions of 24 to 30 hours. The same alloy cast under standard vacuum procedures heretofore in use and tested under these same conditions of 1200 F. and 65,000 p.s.i. shows an average rupture life in the range of between 12 and 20 hours.

Although I have described a preferred embodiment of the present invention, it will be understood that the description is intended to be illustrative, rather than restrictive, as details may be modified or changed without departing from the spirit or scope of the invention.

I claim:

1. In a method of casting metals, the steps comprising pretreating a refractory mold into which molten metal will be transferred from a melting crucible by heating the mold while maintaining a vacuum of less than 20 microns to a temperature in the range between the melting temperature of metal to be poured and the slump temperature of the mold for a period in the range between about 5 minutes and about 120 minutes, cooling the treated mold while maintaining vacuum thereon to a temperature at which there is a solidification gradient between the cooled heat treated mold and the metal to be poured, pouring metal melted under vacuum into the cooled mold while the mold and melt are maintained under vacuum and maintaining vacuum on said mold until the cast metal has solidified.

2. The method according to claim 1 in which the metal fed in molten form to the heated mold is a nickel base alloy.

3. The method according to claim 1 in which the metal fed in molten form to the heated mold is a cobalt base alloy.

4. The method according to claim 1 in which the metal fed in molten form to the heated mold is an iron base alloy.

5. In a method of casting metals, the steps comprising pretreating a refractory mold into which molten metal will be transferred from a melting crucible by heating the mold while maintaining a vacuum in the range between about 0.01 micron and about 5 microns, to a temperature in the range between the melting temperature of a metal to be poured and the slump temperature of the mold for a period in the range between about 5 minutes and about minutes, cooling the treated mold while maintaining vacuum thereon to a temperature at which there is a solidification gradient between the cooled heat treated mold and the metal to be poured, pouring metal melted under vacuum into the cooled mold while the mold and melt are maintained under vacuum and maintaining vacuum on said mold until the cast metal has solidified.

6. In a method of casting metals, the steps comprising pretreating a refractory mold into which molten metal will be transferred from a melting crucible by heating the mold while maintaining a vacuum of less than 20 microns to a temperature in the range between the melting temperature of metal to be poured and the slump temperature of the mold for a period in the range between about 5 minutes and about 120 minutes, cooling the treated mold while maintaining the vacuum thereon to a temperature in the range between 1000 F. and 2400 F., at which temperature there is a solidification gradient between the cooled heat treated mold and the metal to be poured, pouring metal melted under vacuum into the cooled mold while the mold and melt are maintained under vacuum and maintaining vacuum on said mold until the cast metal has solidified.

7. In a method of casting metals, the steps comprising pretreating a refractory mold into which molten metal will be transferred from a melting crucible by heating the mold while maintaining a vacuum of less than 20 microns to a temperature in the range between the melting temperature of metal to be poured and the slump temperature of the mold for a period in the range between about 5 minutes and about 120 minutes, cooling the treated mold while maintaining the vacuum thereon to a temperature in the range between 1800 F. and 2300 F., at which temperature there is a solidification gradient between the cooled heated treated mold and the metal to be poured, pouring metal melted under vacuum into the cooled mold while the mold and melt are maintained under vacuum and maintaining vacuum on said mold until the cast metal has solidified.

8. A method of casting metals comprising heating a mold into which molten metal will be transferred from a meltin crucible in a chamber to a temperature in the range between the melting temperature of a metal to be poured and the slump temperature of the mold while the mold is subjected to a vacuum in the range between 0.1 micron and 5 microns for a period in the range between about 5 minutes and about 120 minutes, cooling the mold while maintaining the vacuum thereon to a temperature in the range between about 1000 F. and about 2400 F., pouring the metal in molten form into the treated mold while the mold and melt are maintained under vacuum and maintaining vacuum on said mold until the cast metal has solidified.

9. In a method of casting metals, the steps comprising reducing metallic material to a molten mass by heating the same to a temperature between about 2400 F. and about 3000 F. under vacuum, heating a refractory mold into which molten metal will be transferred from a melt ing crucible to a temperature at least equal to the pouring temperature of the alloy and in the range between about 2700" F. and about 3000 F. for a period in the range between about 5 minutes and 120 minutes while the mold is subjected to a vacuum of less than 5 microns, cooling the mold While maintaining vacuum thereon to a temperature in the range between about 1000 F. and about 2400 F., pouring the metal in molten form into the treated mold While the mold and melt are maintained under vacuum and maintaining vacuum on said mold until the cast metal is solidified.

10. In a method of casting metals, the steps comprising reducing metallic material to a molten mass by heating the same to a temperature between about 2400 F. and about 3000 F. while under vacuum of less than 20 microns, heating a refractory mold while maintaining a vacuum of less than 20 microns to a temperature which is between 100 F. and 600 F. higher than the melting temperature of the molten mass for a period in the range'between about 5 minutes and about 120 minutes, cooling the mold while maintaining the vacuum thereon to a temperature in the range between about 1000 F. and about 2400 F, pouring the molten metal into the cooled heat-treated mold while the mold and melt are maintained under vacuum and maintaining vacuum on said mold until the cast metal has solidified.

References Cited in the file of this patent UNITED STATES PATENTS 2,548,897 Kroll Apr. 17, 1951 2,806,271 Operhall Sept. 17, 1957 2,824,794 Hathaway Feb. 25, 1958 2,875,483 Hughes Mar, 3, 1959 2,958,719 Beecher Nov. 1, 1960 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,153,824 Oct0b'er'27, 1964 Tunis L. Holmes I It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patentshould read as corrected below.

Column 4, line 52, for "0.8%" read 0.08% column 6, line 51, for "heated" read heat Signed and sealed this 16th day of March 1965,

I (SEAL) guest ER w. :SWIDEH EDWARD J. BRENNER I Attestihg Offi cer Commissioner of Patents 

1. IN A METHOD OF CASTING METALS, THE STEPS COMPRISING PRETREATING A REFRACTORY MOLD INTO WHICH MOLTEN METAL WILL BE TRANSFERRED FROM A MELTING CRUCIBLE BY HEATING THE MOLD WHILE MAINTAINING A VACUUM OF LESS THAN 20 MICRONS TO A TEMPERATURE IN THE RANGE BETWEEN THE MELTING TEMPERATURE OF METAL TO BE POURED AND THE SLUMP TEMPERATURE OF THE MOLD FOR A PERIOD IN THE RANGE BETWEEN ABOUT 5 MINUTES AND ABOUT 120 MINUTES, COOKING THE TREATED MOLD WHILE MAINTAINING VACUUM THEREON TO A TEMPERATURE AT WHICH THERE IS A SOLIDIFICATION GRADIENT BETWEEN THE COOLED HEAT TREATED MOLD AND THE METAL TO BE POURED, POURING METAL MELTED UNDER VACUUM INTO THE COOLED MOLD WHILE THE MOLD AND MELT ARE MAINTAINED UNDER VACUUM AND MAINTAINING VACUUM ON SAID MOLD UNTIL THE CAST METAL HAS SOLIDIFIED. 